Evolution of a synchronous planet-moon pair due to solar tides. Demise of the synchronous moon that initiated Mars' triaxiality. A possible link to tidalites in Vastitas Borealis and Gale Crater

Abstract

Mars' asymmetric figure with two opposing equatorial elevations stems from a frozen tidal bulge raised by a synchronous palaeo-moon Nerio. Nerio's emergence, via in situ accretion or by capture in the disk's remnants, and its synchronisation with Mars' rotation preceded or was coeval with crust formation. The submoon and antimoon regions hypothetically developed thinner crusts, intensifying the tectonics that further amplified Mars' triaxiality. We study Nerio's orbit stability and demise and its impact on Mars' rotation. Nerio may also have been the cause of the tidalites found in sediments. Stellar tides in the planet shrink the orbit of a planet-moon pair, so their synchronism is stable transiently. At some critical distance from the planet, the moon desynchronises and spirals down, spinning the planet up. Application to Mars and Nerio shows that Mars' spin rate at the desynchronisation moment matches the present rate to the first decimal. The coincidence should not be overinterpreted, as post-desynchronisation evolution included Mars' continued spin-up during Nerio's descent (till its destruction amid the LHB), followed by Mars' despinning by solar tides. Nerio's reaching the Roche limit intact is questionable. Beyond LHB hazards, it would imply Mars' larger spin-up, necessitating up to k2/Q ~ 18 to allow subsequent despinning to the present rate. This value may be high even for shallow oceans. Absent future evidence supporting such elevated k2/Q values, Nerio likely perished during the LHB. This verdict may be reconsidered should new data on Mars' ocean show up. The existence and subsequent demise of Nerio are indirectly confirmed by the presence of tidalites in Vastitas Borealis and Gale Crater. During its post-desynchronisation descent, Nerio was capable of producing tidalites in the ocean. After Nerio's demise, this work may have been continued by its massive remnant